Image intensifier devices multiply the amount of incident light they receive and provide an increase in light output, which can be supplied either to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military applications. For example, image intensifiers are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are employed in security cameras and in medical instruments to help alleviate conditions such as retinitis pigmentosis (night blindness). Such an image intensifier device is exemplified by U.S. Pat. No. 5,084,780 entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING by Earl N. Phillips issued on Jan. 28, 1992 and assigned to ITT Corporation the assignee herein.
Image intensifiers include three main components, namely a photocathode, a phosphor screen and a microchannel plate (MCP) positioned between the screen and the photocathode. The MCP is a thin glass plate having an array of microscopic holes through it. Each hole is capable of acting as a channel-type secondary emission electron amplifier. The cathode detects a light image and changes the light image into an electron image. The MCP amplifies the electron image and the screen changes the electron image back to an light image. Gains up to several thousand can be achieved when the microchannel plate is placed in the plane of an electron image in the intensifier tube. Since each channel in an MCP operates nearly independently of all the others, a bright point source of light will saturate a few channels but will not spread out over adjacent areas. This characteristic of "local saturation" makes tubes more immune to blooming at bright areas.
When an image intensifier tube without EMI protection is operated in an electromagnetic field, such as in the vicinity of an operating high power radio or radar transmitter, the image intensifier suffers degradation in performance. The output often goes black or very bright, or some point in between, depending on the design of the power supply. Increased brightness is distracting and in extreme situations may cause the user to lose all contrast, producing a blank viewing screen. Thus, image intensifier tubes conventionally employ a housing assembly for protection from electromagnetic interference as well as from environmental conditions. For an example of prior art devices employing EMI protective housing assemblies and methods of making, reference is made to U.S. Pat. No. 4,924,080, issued to Joseph N. Caserta et al., on May 8, 1990, entitled ELECTROMAGNETIC INTERFERENCE PROTECTION FOR IMAGE INTENSIFIER TUBE, and assigned to ITT Corporation, the assignee herein.
In the prior art, the housing assemblies for image intensifier tubes which provided EMI protection consisted of a non-conductive (generally plastic) tube which was coated on the exterior surface with a conductive material such as silver, copper, or aluminum to provide good protective shielding. The tube contained an opening for a contact sub-assembly which included a separate positive contact (often gold plated brass) with four ceramic chip capacitors soldered to the edges to filter the image intensifier tube's power supply and short the EMI through the capacitors. This sub-assembly was then soldered to the plated plastic housing. However, significant problems exist with this apparatus and method for providing EMI protection. First, prior to the housing being plated, a chemical masking is painted onto certain areas of the tube in order to prevent adherence of plating to those areas which must remain electrically separated or insulated. The chemical masking used to prevent plating in certain areas can often react with the potting used to secure the image intensifier within the housing so as not to allow a full cure (wet potting), thereby permitting movement of the device within the housing and potential optical misalignment. Furthermore, the use of the complex chemical masking regularly results in electrical failures or shorts due to plating in areas that need to be electrically separated, resulting in both lower yields from housing suppliers and greater rejects at the assembly area. Moreover, the use of a separate positive contact allows for variation in the height of the contact off of the surface of the housing. This is a critical interface dimension, requiring strict control and supervision during the manual assembly process. The use of the separate contact thus requires a complex and time consuming assembly operation by a highly skilled operator in order to achieve proper mounting of the contact. Finally, the use of a gold plated contact to act as a positive contact area coupled with the complex chemical masking process and manual assembly results in high manufacturing costs.
Accordingly, a need exists for a method which allows the housing to be plated without the need for a chemical masking process. Moreover, eliminating the separate gold-plated positive contact and its intricate assembly within the housing to reduce costs and increase manufacturing efficiency is highly desirable.